1. Field of Invention
This invention relates to wireless communications systems. Specifically, the present invention relates to systems and methods for demodulating a quick paging channel employed to facilitate offline processing in a wireless communications system.
2. Description of the Related Art
Wireless communications systems are employed in a variety of demanding applications ranging from search and rescue to Internet applications. Such applications require reliable, cost-effective, and space-efficient communications systems and accompanying wireless phones.
Cellular telecommunications systems, such as Code Division Multiple Access (CDMA) communications systems, are often characterized by a plurality of mobile stations (e.g. cellular telephones, mobile units, wireless telephones, or mobile phones) in communication with one or more Base Station Transceiver Subsystems (BTS""s). Signals transmitted by the mobile stations are received by a BTS and often relayed to a Mobile Switching Center (MSC) having a Base Station Controller (BSC). The MSC then routes the signal to a Public Switched Telephone Network (PSTN) or to another wireless phone. Similarly, a signal may be transmitted from the PSTN to a wireless phone via a base station or BTS and an MSC.
Wireless communications networks often employ various channels, such as paging channels and traffic channels, as disclosed in the IS-95 cellular telephone standard, to facilitate communications between a wireless phone and a BTS. Paging messages are transmitted over a paging channel by a BTS to an associated wireless phone to indicate an incoming call. When a wireless phone detects a paging message, a sequence of service negotiation messages is subsequently transmitted between the wireless phone and an associated BTS to establish a traffic channel. A traffic channel typically supports voice and data traffic.
Conventionally, a wireless phone continuously monitors the paging channel for pages indicative of incoming calls. The receiver of the wireless phone remains on while signal processing circuitry within the-wireless phone demodulates the paging channel to determine if a page was sent. Unfortunately, the receiver draws excess power, which significantly limits phone battery life.
Systems for minimizing wireless phone power consumption are often employed in the wireless phone and/or accompanying network to extend phone battery life and associated standby time. To improve standby time, some newer wireless phones operate in slotted mode. In slotted mode, the receiver of the wireless phone is periodically activated in accordance with predetermined paging slots established in accordance with the IS-95 telecommunications standard. An associated BTS transmits pages during the paging slots. Wireless phone standby time is extended by periodically powering-up the phone receiver and demodulating the paging channel rather than continuously demodulating the primary paging channel as done previously.
Unfortunately, paging channel messages are often long and require extensive processing, which increases phone power consumption and reduces battery life and associated standby time. Furthermore, the design of such systems and the associated paging channels necessitates redundant processing of the lengthy paging channel messages to detect incoming calls. This further reduces phone battery life.
Further increases in phone standby time are achieved via a relatively new addition to the IS-95 telecommunications standard known as offline processing. In a wireless communications network employing offline processing, a pair of quick paging channel (QPCH) symbols is periodically transmitted to the wireless phone. The quick paging channel symbols, i.e., quick pages, indicate the presence or absence of an incoming call to be established on a forthcoming traffic channel (F-CCCH). The QPCH symbols arrive in pairs at 9600 bits per second (bps) or 4800 bps. The time slots at which the QPCH symbols are transmitted from an associated BTS are known by the wireless phone, which periodically powers-up the receiver at corresponding time slots.
In a wireless phone employing offline processing, the wireless phone receiver powers-up, samples the QPCH, then immediately powers-down the receiver and processes the QPCH sample offline (when the receiver is off). Subsequent analysis of the QPCH sample or samples indicates whether the wireless phone should power-up the receiver and demodulate the paging channel to receive an incoming page associated with an incoming call. Use of the QCPH helps minimize receiver activation time and the instances of complete paging channel demodulation, enabling a reduction in wireless phone power consumption and an associated extension in phone battery life. Unfortunately, existing systems and methods for detecting pilot signals and associated multipath signal components required to demodulate the QPCH are often undesirably large, expensive, consume excess power, and are generally inefficient.
Hence, a need exists in the art for an efficient system and method for searching for and detecting pilot signals and associated multipath components required to demodulate the QPCH. There is a further need for a method for selecting appropriate searcher parameters for minimizing required hardware. There is a further need for a space-efficient system that can efficiently demodulate the QPCH channel while consuming minimal power.
The need in the art is addressed by the system for facilitating detection and successful decoding of a Quick Paging Channel (QPCH) of the present invention. In the illustrative embodiment, the inventive system is adapted for use with a wireless communications system that supports a primary paging channel and a quick paging channel. The system includes a first mechanism for detecting a pilot signal associated with the quick paging channel based on a received signal. The first mechanism includes a coherent integrator of a first length and a noncoherent integrator of a second length. The second mechanism determines receiver operating characteristics of the system based on the pilot signal. The third mechanism optimizes the first length and the second length based on the operating characteristics.
In a specific embodiment, the first mechanism includes a CDMA receive chain for receiving the received signal and providing a digital received signal to a sample random access memory in response thereto. The sample random access memory includes a mechanism for sampling the digital received signal at predetermined time slots and providing a sampled received signal in response thereto. An interpolator adjusts the rate of the sampled received signal and provides a rate-adjusted signal in response thereto. The first mechanism includes a searcher that includes the first coherent integrator, a second coherent integrator, and the noncoherent integrator. The searcher further includes a complex despreader/correlator for correlating the rate-adjusted received signal with a pseudo noise candidate code and providing a correlation result in response thereto. The first integrator and the second integrator integrate the correlation result over a predetermined number of chips corresponding to the first length and provide first and second integrated values, respectively, in response thereto. The searcher further includes a mechanism for squaring the first and second integrated values and providing first and second squared values, respectively, in response thereto. The searcher further includes a mechanism for adding the first and second squared values and providing a sum in response thereto. The noncoherent integrator integrates the sum over a predetermined number of values and outputs an estimate or estimates of the pilot signal in response thereto. The predetermined number corresponds to the second length.
In a more specific embodiment, the first mechanism further includes a mechanism for computing a pilot energy associated with the pilot signal from the pilot estimate(s). The second mechanism includes a mechanism for determining the receiver operating characteristics based on the pilot estimate(s) and the pilot energy and a demodulated QPCH channel. The third mechanism includes a mechanism for computing, based on the receiver operating characteristics, optimal values for the first length and the second length and selectively adjusting the first length and the second length in response thereto. The first length is 512 and the second length is less than or equal to 4.
The novel design of the present invention is facilitated by the second mechanism that provides receiver operating characteristics based on a pilot signal and the QPCH channel. The receiver operating characteristics are then employed by the third mechanism to optimize integration length parameters of the searcher, which results in a significant reduction in the requisite size of the sample random access memory.